The present invention relates generally to aluminide coatings, and more particularly to a one-step process for forming diffused noble metal-aluminide coatings.
In the gas turbine engine industry, high temperature corrosion- and oxidation-resistant protective coatings for nickel-based and cobalt-based alloy components, such as blades and vanes, are required. These coatings are particularly useful for new generation gas turbine engines that are designed to operate at higher turbine inlet temperatures for greater engine performance and fuel efficiency.
Diffused aluminide coatings have been used to protect alloy components in the turbine section of gas turbine engines. In general, a diffused aluminide coating is formed by applying an aluminum-based powder to an alloy substrate and heating it to diffuse the aluminum into the substrate.
Diffused aluminide coatings may include chromium or manganese to increase their hot corrosion/oxidation resistance. Furthermore, addition of noble metals, such as platinum, to provide platinum-aluminide coatings, has markedly improved hot oxidation resistance. However, formation of these modified aluminide coatings requires additional processing steps and more complex diffusion heat treatment regimes.
Specific examples of known coating processes include providing a platinum-enriched aluminide surface by electroplating a thin film of platinum onto a carefully cleaned alloy substrate, overaluminizing the platinum thin film by applying an activated aluminum-bearing coating via pack cementation, CVD, thermal spray or other known application methods, and then heating the coated substrate at a temperature and for a time sufficient to form the platinum-enriched diffused aluminum coating. Optionally, the platinum may be diffused into the substrate either prior to or after the application of the aluminum. It is also known to form the modified diffused aluminide coating by employing a sequential two-step electrophoretic deposition process with a diffusion heat treatment following each electrophoretic deposition step. (See U.S. Pat. No. 5,057,196 to Creech et al., which is incorporated by reference herein.)
All of the known prior art processes use a multi-step application procedure to sequentially apply a platinum-enriched layer and an aluminum-bearing layer followed by diffusion heat treatment to provide modified noble metal-aluminide coatings. These multi-step processes are expensive and time consuming, and make the application of such coatings less advantageous from a commercial viewpoint. While the cost of noble metals and chromium group metals included in the modified aluminide coatings constitute a significant cost of the coatings, the costs associated with the processing methods are an equally significant if not greater cost of the coatings.
A need therefore exists for methods to streamline the noble metal-aluminide coating processes to improve efficiency, decrease cost and provide an effective corrosion- and/or oxidation-resistant protective coating for nickel or cobalt-based alloy substrates. The present invention addresses that need.
Generally describing the present invention, a xe2x80x9cone-stepxe2x80x9d method of forming a diffused aluminide coating is provided. With the inventive method, two or more powdered metals or metal alloys are applied and diffused into the metal substrate together, preferably using a multi-stage heating process. This method contrasts with the prior art technology where powdered metals were applied and diffused into the substrate separately.
A variety of powdered metals may be applied with the inventive one-step method. In general, the coating compositions preferably comprise a mixture of: (1) a platinum powder, and (2) an aluminum-bearing component. The platinum powder preferably includes silicon either as a pre-alloy powder or an alloy powder. In one embodiment, the aluminum-bearing component includes an aluminum alloy powder comprising aluminum and chromium, although manganese is also added in some preferred embodiments. In other preferred embodiments, an aluminum powder is used either in addition to, or in place of, the aluminum alloy powder. In yet other preferred embodiments, hafnium, yttrium and/or lanthanum are added to one of the aforementioned powders, or are added to the green coating composition separately.
In the coating composition described above, a portion or all of the platinum in the platinum powder can be replaced by other noble group metals, for example, palladium, ruthenium, and rhodium.
Regardless of the metals used, the inventive one-step method diffuses all of the metals into the substrate together. To do that, a multi-stage heating process is preferably employed. With the multi-stage heating process, the powder-covered substrate is initially heated to a first temperature to begin the diffusion process, and is then heated to a second temperature to complete the diffusion. In some embodiments a pre-diffusion heat treatment is also used.
One object of the present invention is to provide a simple, economical method of providing diffused noble metal-aluminide coatings.
Further objects and advantages of the present invention will be apparent from the description provided below.